From: mharris AT blackwidow DOT saultc DOT on DOT ca
Date: Fri, 31 Jan 1997 16:52:03 -0500 (EST)
Reply-To: mharris AT blackwidow DOT saultc DOT on DOT ca
To: Benjamin D Chambers <chambersb AT juno DOT com>
cc: djgpp AT delorie DOT com
Subject: Re: fixed point math: NEED HELP QUICK!
In-Reply-To: <19970128.181621.14527.2.chambersb@juno.com>
Message-ID: <Pine.LNX.3.95.970131164737.140G-100000@capslock.com>
Organization: Total disorganization.
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On Tue, 28 Jan 1997, Benjamin D Chambers wrote:

> 
> On Tue, 28 Jan 1997 20:30:34 -0500 DJ Delorie <dj AT delorie DOT com> writes:
> >
> >> function.  Anyway, here's my problem;  To my understanding the 
> >following
> >> are 100% equal:
> >> 
> >> other_stuff = stuff / 256;
> >> other_stuff = stuff << 8;
> >
> >The second multiplies.  Try ">> 8" instead.
> >
> >
> Also, just to be clear, the first gets translated into the second when
> compiling with optimizations on (-O2).
> In fact, I believe *ALL* integer multiplication/division by a constant
> gets converted to shifts when using -O2, so you might as well use the
> first - if for nothing else then for readability's sake.

If *ALL* integer mults/divs are converted to shifts, then what would
be the equivalent for:

other_stuff = stuff * 57
other_stuff = stuff << ???

The point I'm trying to make is that no, only mul/div's by powers of 2
would be converted to shifts.  Others such as multiplying by numbers
whose factors are of powers of 2 can also be optimized in this way.
Numbers that don't fit (such as 57) can't be implemented via shifting
and be optimal.


Mike A. Harris        |             http://blackwidow.saultc.on.ca/~mharris
Computer Consultant   |    My webpage has moved and my address has changed.
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